Curable thermosetting cyanate ester composition

A thermosetting resin composition comprising a thermosetting cyanate ester resin composition (A) and a butadiene based copolymer (B) (i) or an epoxy resin-modified butadiene based copolymer (B) (ii), in which said components (B) (i) or (B) (ii) are used for modifying said component (A);

• component (A) being:
• (1) a cyanate ester thermosetting resin comprising a cyanate ester compound selected from the group consisting of
• (i) a polyfunctional cyanate ester having at least two cyanate groups in its molecule,
• (ii) a prepolymer of the cyanate ester of (i),
• (iii) a copolymer of the cyanate ester of (i) and an amine, and
• (iv) mixtures thereof,
• said cyanate ester being represented by the formula
• R(OCN).sub.m ( 1)
• wherein R is an aromatic nucleus-containing residue having 1-10 benzene rings derived from an aromatic hydrocarbon selected from the group consisting of benzene, biphenyl and naphthalene, residue derived from a compound in which at least two benzene rings are bonded to each other by a bridging member selected from the group consisting of ##STR4## wherein R.sup.1 and R.sup.2 are the same or different and each represents a hydrogen atom or an alkyl group containing 1 to 4 carbon atoms, ##STR5## said aromatic nucleus is non-substituted or is substituted with a substitutuent selected from the group consisting of alkyl groups containing 1 to 4 carbon atoms, alkoxy groups containing 1 to 4 carbon atoms, chlorine and bromine;
• and m is an integer of at least 2;
• wherein the cyanato groups are always directly bonded to the aromatic nucleus
• (2) a cyanate ester-maleimide resin composition,
• (3) a cyanate ester-maleimide-epoxy resin composition, or
• (4) a cyanate ester-epoxy resin composition;
• the amount of the cyanate ester compositions (2)-(4) being more than 50% by weight, and
• Component (B) being as follows:
• (i) a butadiene based copolymer comprising a butadiene-aromatic vinyl monomer copolymer having 0.5 or more functional radical selected from the group consisting of carboxyl radicals, hydroxyl radicals, acid anhydride radicals, melcupto radicals, epoxy radicals and amino radicals, and having a melting point of 10.degree. C. or more, and wherein the 1,2-type content in the butadiene structure of the copolymer chain is 50% or more and wherein, the weight ratio of butadiene to aromatic vinyl monomer in the copolymer is in the range of 80:20 to 10:90;
• or
• (ii) an epoxy resin-modified butadiene based copolymer comprising said butadiene based copolymer (B) (i) having a carboxyl radical modified with a polyfunctional epoxy resin, wherein said copolymer (B) (ii) has a melting point of 10.degree. C. or more and an epoxy equivalent of 300-8,000, the ratio by weight of component (A) to component (B) being in the range of 95:5 to 30:70.